Researchers struggle to find the most efficient—and least expensive—way to bring solar energy to consumers even when the sun isn't shining.

While engineers build cheaper and more efficient solar panels to soak up more of the sun's rays, it's storage that needs a breakthrough so that solar energy can be used when the sun's not shining. Batteries, at least those we have today, just aren't cutting it. "We need to find a way to store massive amounts of electrical energy," says Michael Aziz, a professor of energy technologies at Harvard University. "That's the single biggest obstacle to getting a large fraction of our electricity from solar power."

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Flow Batteries

Aziz and his colleagues turned to fuel cells for inspiration. Fuel cells power space capsules and aircraft, capitalizing on reactions that convert the chemical energy in small organic molecules like methanol into electricity. Aziz figured that if he could craft a fuel cell that also runs in reverse—essentially converting energy back into chemical reactants—the resulting flow battery could store solar power using inexpensive, organic fuel.

Inorganic, metal ion-based flow batteries have been in use since the 1980s. These older models were constructed as tanks filled with vanadium ions, and could be customized to deliver more hours of energy by simply increasing the amount of vanadium in a storage tank. Unfortunately, vanadium isn't cheap.

"If all you have to do is increase the size of the storage chemicals, that's fine—until you have to pay for the chemicals," Aziz says.

His team found a less expensive alternative in the form of quinones—small organic molecules that help rhubarb plants store energy. "In photosynthesis, quinones are known to recharge over and over again with high efficiency," Aziz says. While vanadium flow batteries cost almost 2 cents per kilowatt-hour, Aziz says that his quinone-based flow batteries may ultimately cost as little as 1/4 of a cent per kilowatt hour.

Hydrogen

But quinone flow batteries are not the only recent advances in the struggle to store sunlight. "There are lots of clever people doing lots of clever things in the battery business," says Tom Meyer, a professor of chemistry at the University of North Carolina.

Last week, Meyer announced a new strategy—converting solar energy to hydrogen, instead of electricity. The key to Meyer's method is water: "You start out with water and just break it into its elemental form," he says, "then collect that hydrogen in a tank and then burn it at night."

Although Meyer's team is currently working with water, he hopes to extend his project to power homes while reducing our carbon footprint. Meyer's ambitious plan involves using solar energy to convert carbon dioxide into methanol, its combustive cousin. At night, a power plant would burn that methanol as fuel, converting it back into carbon dioxide, which it would capture and store for later. The next time the sun came out, the process would begin again, effectively recycling carbon—and potentially reducing harmful emissions.

"It's still a research project," Meyer cautions. "But it's knocking on the door to go to the next stage."

Salt

Other methods of storing solar power for a rainy day involve converting the sun's energy into heat, which is then captured in thermal storage tanks. Abengoa, a renewable energy firm based in Spain, has already built several solar plants that store excess energy in molten salt, which can absorb extremely high temperatures without changing state. Abengoa recently secured yet another contract to build a salt-based 110 mega-watt solar storage plant in Chile, which should be able to store 17 hours of energy in reserve.

Between these novel technologies for storing sunlight, will solar power eventually negate the need for fossil fuels? "It's hard to imagine it going down to zero," Aziz says. "[But] I foresee a future where we can use this technology to vastly cut down on fossil fuel use," he says.